Embodiments of the invention relate generally to a redundant array of independent tapes, and more specifically, to systems and method for protecting stored data utilizing a time multiplexed redundant array of independent tapes.
In general, redundant array of independent disks (RAID) is a storage technology that combines multiple physical storage devices into a logical unit(s) for the purposes of data redundancy and performance improvement. In RAID systems, data is distributed across the physical storage devices in one of several ways, which are referred to as RAID levels, depending on the specific level of redundancy and performance required. In traditional RAID systems, all of the physical storage devices are accessible at the same time for write operations. Thus, a RAID stripe may be written essentially in parallel and any remapping that might occur, such as due to a newly detected defect, can be handled dynamically by remapping the block or stripe data with little impact on performance.
While some physical storage devices, such as hard disks and flash memory devices have somewhat variable capacity, the exact capacity of these devices is known prior to their use. For example, many of these types of devices are formatted at the factory and contain controllers that report capacity information to their host machine and the capacity information is available to a RAID controller before the RAID stripes are laid out. Accordingly, the RAID controller can ensure that all RAID stripes are written completely by selecting a stripe count such that the capacity required is no greater than the available capacity of the smallest disk.
In some physical storage devices, such as tapes, the capacities of the storage devices are not known prior to their use. For example, the tapes may have different physical lengths as well as different defects, which impact the capacity of the tape. Since data is written sequentially on a tape, the data capacity of a tape may not be known until the write operation to that tape is complete. In addition, write errors may be encountered during the write operation that results in a loss of capacity for the tape. In many cases, it is more cost effective to continue writing following a write defect than to take the time to attempt to re-write the location.